Veneers

Veneers, thin sheets of wood cut with the grain parallel to the surface, can vary in thickness, before use, from 0.25 mm up to about 6.4 mm. All decorative veneers produced in Europe are cut into 0.6-0.9 mm for face quality, average thickness 0.7 mm (1/40 in). In the United States and Australia most veneers produced are cut 0.9 mm (1/28 in). They are produced by three basic methods, sawing, rotary peeling and slicing. Those of high value or striking appearance are used for surface decoration but veneers are also used structurally to make plywood boards and to make bent laminated and moulded shapes that could not otherwise be achieved in a single thick piece of wood without chemical modification. The original method of production by sawing has been replaced in all but a very few cases by knife cutting for commercial production. Logs intended for slicing are cut open and examined for figure, grain, texture and defects before cutting into suitable sections for veneer production. The halved, quartered, or flitched log sections are then usually steamed or soaked in hot water for one to several days before cutting. Logs intended for rotary peeling are treated in the round.

In rotary cutting, the log is held at either end in a large lathe and rotated against the knife, which is moved forward at each rotation to preserve the thickness of the cut. This method of cutting is specially suited for high volume production of large sheets of veneer used to make plywood. However, it is also used in one or other of its five basic variations to produce many freak figured decorative veneers, such as bird's eye maple, figured betula and masur birch, that cannot be extracted by any of the other cutting methods.

In slicing, a flitch of wood held in a frame is moved against a knife. Four different methods of cutting and mounting the flitch are used to extract the best figure. Veneer slices are kept in sequence during subsequent drying operations so that the sequence of a figure from one leaf to another is preserved. They are usually sold in this form as a flitch that may subsequently be broken up and sold in books or individual leaves or sheets.

Whether rotary peeling or slicing is used, the cutting action is essentially similar in both. As the knife separates the veneer from the flitch, the separated layer of wood is severely bent and stresses build up in the region near the knife edge. When these stresses exceed the strength of the wood, failure occurs resulting in a series of knife checks across the side of the veneer that was against the knife. This side of the veneer is called the loose side and the side away from the knife is called the tight side. By using a pressure bar to restrain the veneer as it is cut, it is possible to substantially reduce this problem and in some cases to eliminate it. The extent to which this is possible is determined by the species, by the thickness of cut, by the setting up of the machine and by the temperature of the wood. Diffuse porous hardwoods with well-distributed rays, such as birch, are more likely to yield tight veneers than ring porous hardwoods such as oak.

Holding a piece of veneer with one edge in each hand and flexing it will usually reveal tight and loose sides; veneers feel stiffer when flexed to close the checks and more limp when they are flexed open. If the veneer feels the same in both directions it is probably tight. Knife checks often show up months or years after the veneer has been laid, as parallel-to-grain cracks through the finish. Whenever possible therefore the tight side (also called the face side) should be laid face up and great care should be taken not to sand through it during finishing. A comprehensive review of all aspects of the preparation, selection and use of veneers is given by Lincoln (1984).

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